docs: def storage_list(storage_name=None):
      """List the storage IDs for the unit"""
      _args = ['storage-list', '--format=json']
      if storage_name:
          _args.append(storage_name)
      try:
          return json.loads(subprocess.check_output(_args).decode('UTF-8'))
      except ValueError:
          return None
      except OSError as e:
          import errno
          if e.errno == errno.ENOENT:
              # storage-list does not exist
              return []
          raise